The invention relates to waterborne sanitary can coating compositions, to coated cans prepared with them and to a process for their preparation.
More in particular the invention relates to water soluble can coating compositions, derived from combinations of epoxy resins and phenol- or melamine-formaldehyde resins as curing agent.
The rather attractive coating properties of coating films derived from such combinations, such as good chemical resistance, good adhesion and corrosion resistance, are known for a long time already.
Can coating compositions for feed and beverages comprising these combinations were known from e.g. EP 0511223A1; U.S. Pat. No. 4,758,638A; U.S. Pat. No. 4,722,982A; JP 63142068A; JP 6109737A, disclosing organic solvent based can coating systems containing from 30 to 40 wt resin solids but having as main disadvantage in respect of health and environment the evaporation of large amounts of solvents after and/or during application. On the other hand water borne can coating systems were disclosed in e.g. EP 0116225B1; EP 0367493B1; U.S. Pat. No. 5,387,625A; U.S. Pat. No. 4,021,396A; U.S. Pat. No. 4,308,185A; U.S. Pat. No. 4,212,781; U.S. Pat. No. 4,302,373; U.S. Pat. No. 4,413,015; U.S. Pat. No. 4,683,273A; U.S. Pat. No. 4,897,434A; EP 0054290B1; EP 0006334B1, comprising the epoxy resin component incorporated into a prepolymer, bearing a functional group, e.g. carboxyl group, and derived from in addition to the epoxy resin, e.g. acrylic acid or methacrylic acid, and esters of these acids and other vinyl comonomers such as styrene.
The remaining carboxyl groups of the precopolymer are neutralized with amine to form a water soluble or water dispersable precopolymer in the coating composition. However an important disadvantage of said water borne coating compositions was that they were not suitable for internal food can coatings due to their bad sterilization resistance.
Efforts to develop water borne binders for can lacquers through modification of epoxy resins with amino acids were known from e.g. W T Raudenbusch, A Noordam, W J van Westrenen, FATIPEC 1988 (Aachen).
It will be appreciated that there is still a growing need for water borne can coating systems, providing a good combination of coating film properties as obtained earlier with epoxy/phenol formaldehyde resin systems (high performance resins) in combination with a good sterilization resistance, and which does not produce significant amounts of solvents and does not release low molecular toxic epoxy containing compounds during or after curing, due to the increasing pressure of environmental and health authorities (health/environment friendly systems).
It is therefore an object of the present invention to provide said waterborne sanitary can coating compositions, i.e. waterborne can coating compositions for food or beverage cans, which must have sterilization resistance.
Another object of the present invention is to provide a process for the preparation of said waterborne coating compositions.
As a result of extensive research and experimentation said waterborne sanitary can coating compositions aimed at could surprisingly be found.
Accordingly the invention relates to waterborne sanitary can coating compositions derived from at least
(a) an epoxy resin, derived from phenol derivatives such as tetraphenylol ethane or para tert.butyl phenol, bisphenol, and epichlorohydrin, which has been converted into a cationic NH2 containing form by reaction with an amine, an amine adduct or ammonia and has been subsequently converted in the corresponding ammonium salt by addition of an organic acid,
(b) a phenol-formaldehyde resin,
(c) a polyketone resin, having a number average molecular weight Mn of at least 500, and
(d) water, containing minor amounts of organic solvents.
Preferably the invention relates to waterborne sanitary can coating compositions wherein the equivalent ratio between the starting epoxy resin and secondary amine is in the range of from 0.9:1 to 1.1:1, and more preferably as close as possible to 1:1, wherein the weight ratio between phenol-formaldehyde (b) and epoxy resin (a) is from 10:90 to 35:65, and wherein the equivalent weights of the polyketone resin and primary amine groups, i.e. the amount of polyketone required for formation of one pyrrole ring with one primary amine group, are stoichiometric, and wherein the amount of added organic acid will be sufficient to render the cationic resin water (containing cosolvents) soluble or dispersable to obtain a stable composition.
More preferably the invention relates to waterborne sanitary can coating compositions comprising at least
(a) an epoxy resin derived from bisphenol and epichloro-hydrin, which is solid at room temperature, has a melting range in the range of from 100 to 150xc2x0 C., a hydroxyl content of from 3400 to 4000 mmol/kg and preferably from 3400 to 3800 mmol/kg, an epoxy group content (EGC) of from 800 to 250 mmol/kg and an epoxy molar mass (EMM) in the range of from 1290 to 3200 g, and which has been converted into a cationic NH2 containing form by reaction with an amine, an amine adduct or ammonia, and has been subsequently converted in the corresponding ammonium salt by addition of an organic acid,
(b) a phenol-formaldehyde resin,
(c) a polyketone resin, having a number average molecular weight Mn in the range of from 500 to 10,000, and
(d) water, containing minor amounts of organic solvents.
With the term xe2x80x9cwaterborne sanitary can coating compositionsxe2x80x9d as used throughout the specification and claims, are meant can coating compositions, which are water soluble or dispersable and which can be successfully used for cans containing food or beverages, which have to be sterilized after packing.
With the term xe2x80x9cminor amounts of organic solventsxe2x80x9d is meant, that the organic solvents can occur in a proportion of at most 25 wt % and preferably at most 15 wt % relative to the weight of the aqueous solvent.
Such organic solvents may be selected from 2-butoxyethanol-1, butanol, ethanol, propanol, ethylene glycol monoethyl ether, isopropanol, propylene glycol, monomethyl ether, Butyl CELLOSOLVE solvent, Butyl CARBITOL solvent and mixtures thereof (CELLOSOLVE and CARBITOL are trademarks). The use of 2-butoxy ethanol-1 is preferred.
It will be appreciated that in the epoxy resin reaction product (a) must be present at least one primary amino group. Preferably the epoxy reaction product contains no secondary nitrogen groups, and most preferably the majority of the amino groups are primary in the epoxy reaction product (a).
It will be appreciated that the amount of amine source to be used must be at least an amount which is sufficient to render the resin cationic and therefore water soluble or dispersable after neutralization with organic acid and such that after addition of component (c) all amino groups introduced have incorporated in formed pyrrole rings in the cured coating films.
The waterborne sanitary can coating compositions of the present invention preferably comprise a component (c), which has a number average molecular weight of from 1000 to 4000.
More preferably said number average molecular weight of component (c) is in the range of from 1200 to 3000 and most preferably in the range of from 1500 to 2500.
Preferred waterborne sanitary can coating compositions of the present invention can be characterized by an equivalent ratio between the epoxy resin and the secondary amine 0.95:1 to 1.05:1 and more preferably as close as possible to 1:1 and the weight proportions of the components (a) and (b) in the range of from 28:75 to 32:68 and molar proportions of the component (c) and ammonia or amine in the range of from 1:1 to 1:2.
As organic acid can be used for the solubilisation or dispersion of the converted epoxy resin relatively low molecular weight carboxylic acids, such as formic acid, acetic acid, lactic acid, of which lactic acid is preferred.
As component (b) preferably is used a phenol/formaldehyde resin of the resol type, obtainable from phenol or substituted phenol with molar excess formaldehyde under basic reaction conditions.
Commercial representatives of suitable phenol-formaldehyde resins are BAKELITE 100; SANTOLINK EP 560; PHENODUR PR 217; PHENODUR PR 401; PHENODUR PR 285; PHENODUR PR 722; EPIKURE DX-200; UCAR CKS-3892; URAVAR FN-1, FB 110, FB 120, FB 190, FB 209 resins (BAKELITE, SANTOLINK, URAVAR, PHENODUR, UCAR and EPIKURE are trademarks).
Preferably PHENODUR PR 217, PHENODUR PR 285 or EPIKURE DX-200 resins are used as component (b).
Component (a) is preferably selected from solid epoxy resin having an EGC in the range of from 250 to 650 and an EMM in the range of from 1700 to 3100 and a melting range in the area of from 100 to 115xc2x0 C., which is prereacted with an amine or amine adduct into a cationic NH2 containing form and has been subsequently converted in the corresponding ammonium salt by addition of an organic acid.
The amines are more preferably selected from the group of aliphatic di- or poly-amines of from 2 to 20 carbon atoms inclusive.
Preferred are those amines, having from 2 to 12 amino groups inclusive, at least one of which is primary amino groups, and more preferred are those amines, having from 2 to 6 carbon atoms. More preferably the amines have at least two primary amino groups.
In general, those amines are suitable, which, after reaction with the epoxy resin, are still capable to readily react with the polyketone to form pyrrole rings. More in particular those amines are formed by adducts of a starting amine and a ketone, such as acetone, methylisobutyl ketone, methylpropyl ketone, of which methylisobutyl ketone is preferred.
The starting amines which can be used can be selected from dialkylene triamine (e.g. diethylene triamine; N,N-dimethyldipropylene triamine; ethylenepropylene triamine; dipropylene triamine) or trialkylene tetramine (e.g. triethylene tetramine; N,Nxe2x80x2-bis(3-aminopropyl)-ethylene diamine), N-ethyl-1,2-ethane diamine, N-methyl-1,3-propane diamine, N-cyclohexyl-1,3-propane diamine, 2[(2-aminoethyl)amino]ethanol, 1-piperazineethane amine, N(2-aminoethyl)1,3-propane diamine, N(3-aminopropyl)1,3-propane diamine, N,Nxe2x80x3-1,2-ethanediylbis-(1,3-propane diamine, Nxe2x80x2-(3-aminopropyl), N,N-dimethyl-1,3-propane diamine.
It will be appreciated that, if desired, mono secondary amines can be used as end capping agent for remaining epoxy groups in initially prepared amine derivatives of starting epoxy resins. Such end capping agents can be selected from diethyl amine, dimethyl amine or other mono secondary amines.
It will be appreciated, that other aspects of the present invention are formed by coated tin plate coil, obtainable by application of the hereinbefore specified waterborne coating compositions by spraying or roller coating, and by coated sanitary cans for sterilised food or beverages, obtained from coated tin plate coil.
It has been found that attractive properties of the aqueous can coating compositions can only be reached, if a prereaction occurs between the starting epoxy resin and the amine, amine adduct or ammonia.
Therefore another aspect of the present invention is formed by a process for the preparation of waterborne sanitary can coating compositions comprising:
(i) the reaction of an epoxy resin with an amine, amine adduct or ammonia to form a cationic NH2 containing epoxy resin,
(ii) reaction of the cationic NH2 containing epoxy resin and an organic acid in water to form a water soluble or dispersable resin,
(iii) addition of the phenol-formaldehyde resin and the polyketone resin to the water soluble or dispersable resin.
The invention is illustrated by the following examples, however without restricting its scope to these embodiments.
The following Evaluation Methods have been used in these examples.